Glasses and glass-ceramics of varying base ingredients, e.g., silicates, aluminosilicates, borosilicates, and phosphates, and containing iron oxide in substantial amounts are known to the art. When the batch materials are melted under oxidizing or neutral conditions, the glasses resulting therefrom can exhibit magnetic behavior, the magnitude of such behavior depending upon glass composition, annealing schedule, etc. When subsequently subjected to the proper heat treatment, however, minute crystals structurally similar to magnetite (Fe.sub.3 O.sub.4) are developed and/or caused to grow in size within the glassy matrix and the magnetic behavior then displayed by the articles is significantly enhanced. The presence of appreciable magnetization in such articles is evidenced by the fact that small pieces thereof can be lifted with an ordinary laboratory permanent magnet. Also, when such articles are placed in the proximity of a magnetic compass, the needle thereof is deflected, thereby indicating that the earth's field is sufficient to induce perceptible magnetization in the glass. Two recently-issued patents disclosing such bodies are U.S. Pat. Nos. 4,043,821 and 4,126,437.
U.S. Pat. No. 4,043,821 is drawn to the production of glass-ceramic products having compositions, expressed in weight percent, of about 34-40% SiO.sub.2, 2-6% Al.sub.2 O.sub.3, 2-6% B.sub.2 O.sub.3, 40-50% SiO.sub.2 +Al.sub.2 O.sub.3 +B.sub.2 O.sub.3, O-10% MgO, 24-40% CaO+MgO, 16-30% Fe.sub.2 O.sub.3, and 0.7-2% Cr.sub.2 O.sub.3 as the nucleating agent. The precursor glass bodies were crystallized in situ to glass-ceramic articles via heat treatment at temperatures between 850.degree.-1000.degree. C. The finished products were designed for excellence in resistance to bending and abrasion. Accordingly, the starting compositions and heat treatments were fashioned to favor the crystallization of wollastonite (CaO.multidot.SiO.sub.2) and hedenbergite (CaO.multidot.FeO.multidot.2SiO.sub.2), with rankinite (3CaO.multidot.2SiO.sub.2) and magnetite (Fe.sub.3 O.sub.4) being present as minor phases. The inclusion of alkali metal oxides is statedly to be avoided.
U.S. Pat. No. 4,126,437 is directed to glass compositions suitable as electrostatographic carrier materials exhibiting high magnetic permeabilities together with superparamagnetism. The single exemplary composition provided consisted, expressed in weight percent, of about 26.5% SiO.sub.2, 22.6% B.sub.2 O.sub.3, 15.3% CaO, 10.7% Al.sub.2 O.sub.3, and 24.9% Fe.sub.2 O.sub.3. The patent teaches melting the batch materials at temperatures up to 1350.degree. C. and then quenching the melt to a glass. X-ray diffraction and electron microscopic analyses of the glass evidenced no trace of crystallinity therein. However, transmission electron microscopy, density, and magnetic measurements indicated the presence therein of microinhomogeneities in the form of amorphous phase separation. Such phase separation appeared to have dimensions on the order of about 100 A and to consist of a continuous iron-depleted phase with islands of an iron-rich phase. Magnetic measurements determined that the iron-rich phase was composed of amorphous clusters of anti-ferro-magnetically coupled Fe ions. The bulk glass displayed paramagnetic behavior at ambient temperatures, due primarily to the nonclustered Fe ions in the glass.
The quenched glass was thereafter crushed to a particle size between about 100-200 mesh and the particles heat treated at about 600.degree.-800.degree. C. for times up to 24 hours to cause the growth of extremely minute Fe.sub.3 O.sub.4 crystallites therein. In the preferred embodiment of the invention, the crystallites had sizes up to about 500 A, thereby manifesting superparamagnetic behavior.
Glass and crystal-containing particles which exhibit magnetism would have tremendous potential utility as supports for biological materials employed in assays or reactors. (As used herein, the terms "magnetism" and "magnetic" will refer to materials which are either ferri-, ferro-, or superparamagnetic.) For example, in current practice an antibody, enzyme, or other protein is chemically bonded to a support which serves to immobilize the antibody, enzyme, or other protein. The supporting material is commonly in the form of powdered glass or an inorganic crystalline material which may be porous, if desired. The resulting composite body, i.e., the support plus the attached biological, is suspended in a liquid and reacted with reagents present therein. At some stage in the process, however, it becomes necessary to separate the composite body from the liquid. Centrifugation and/or filtering have customarily been utilized to accomplish this. Where magnetic particles are utilized, however, the application of an external magnetic field can be used, for example, to position the particles at a particular location within the reaction vessel while the liquid is decanted or aspirated away. This capability self-evidently allows the separation to be undertaken more conveniently than through centrifugation. Furthermore, magnetic separation techniques can discriminate against spurious non-magnetic particles, thus providing a more exacting and economical separation than is possible through centrifugation. High gradient separation techniques can be used permitting quite weakly magnetic particles to be separated from liquids and can be very useful in expediting separation.
The concept of employing magnetic materials as supports for assay purposes is not novel in itself, the use of Fe.sub.3 O.sub.4 for this purpose being proposed in the literature. Nevertheless, that proposal has not been without problems. For example, finely-divided magnetite particles exhibit agglomeration even in the absence of a magnetic field. Agglomeration inhibits dispersion of the particles through the liquid volume, thereby slowing the reaction between the particles and the reagents in the liquid. A further factor militating against the widespread use of Fe.sub.3 O.sub.4 as a support medium is its intrinsic lack of chemical durability when exposed to the various liquids commonly encountered in such assays. Finally, a porous support medium is normally desired since such provides greater surface area for the immobilization of biological materials thereon.
U.S. Pat. No. 4,140,645 discloses the preparation of glass and glass-ceramic articles which demonstrate efficient and uniform heating when placed in an oscillatory magnetic field and which articles contain magnetite crystals. The articles have overall compositions, expressed in weight percent on the oxide basis, selected from the groups of:
(a) 2-10% Na.sub.2 O and/or K.sub.2 O, 5-20% B.sub.2 O.sub.3, 15-40% FeO, 0-32% Al.sub.2 O.sub.3, and 35-65% SiO.sub.2 ; and PA1 (b) 1.5-6% Li.sub.2 O, 10-40% FeO, 10-20% Al.sub.2 O.sub.3, 45-66% SiO.sub.2, 0-5% TiO.sub.2 and/or ZrO.sub.2, and 0-5% B.sub.2 O.sub.3, at least 1% B.sub.2 O.sub.3 being required when the proportion of FeO is less than 15%.
There is no discussion of forming porous bodies of such compositions.